Apparatus for generating a plurality of beamlets

ABSTRACT

The invention relates to an apparatus for generating a plurality of charged particle beamlets, comprising a charged particle source for generating a diverging charged particle beam, a converging means for refracting said diverging charged particle beam and a lens array comprising a plurality of lenses, wherein said lens array is located between said charged particle source and said converging means. 
     In this way, it is possible to reduce aberrations of the converging means.

The present patent application is a Divisional of application Ser. No.10/797,364, filed Mar. 10, 2004 now U.S. Pat. No. 7,129,502, whichclaims the benefit of U.S. Provisional Application No. 60/453,745, filedMar. 10, 2003 and International Application No. PCT/NL2004/0070174.

BACKGROUND

The present invention relates to an apparatus for generating a pluralityof charged particle beamlets. Charged particle beams are used in avariety of systems, like lithography and microscopy systems. Some ofthese systems use a single source generating one beam which issubsequently split into a plurality of beamlets. The charged particlesource used in these systems emits a charged particle beam with adefined opening angle, i.e. a diverging beam. The diverging beam oftenneeds to be collimated, i.e. transformed into a parallel beam. In mostapplications a lens or lens assembly is used to refract the divergingbeam emitted.

However, the change of angle of a charged particle beamlet is notexactly defined due to so-called chromatic aberrations. As a result thespot size of the beamlets on the target to be exposed or imaged alsoincreases.

In GB2340991 and in U.S. Pat. No. 5,834,783, U.S. Pat. No. 5,905,267,U.S. Pat. No. 5,981,954, U.S. Pat. No. 6,124,599, U.S. Pat. No.6,137,113, U.S. Pat. No. 6,166,387, U.S. Pat. No. 6,274,877, and theJournal of Vacuum Science and Technology B18 (6) pp. 3061-3066, chargedparticle beam lithography systems are disclosed, comprising a lensassembly for refraction of a diverging beam into a parallel beam. Afterrefraction, the beam is split up into a plurality of beamlets using anaperture array. In these lithography systems an image of the aperturesis projected on the surface to be exposed. To decrease the spot size,the image is reduced by a factor 200. The aberrations are dominated bythe aberrations of the last lens in the system and not by theaberrations of the collimation assembly. However, these aberrations willaffect the performance of the system.

The lens assembly in such systems has a chromatic aberrationΔβ=C·ΔE/E·β, wherein β is the angle of an incoming ray with respect tothe normal of the lens assembly and E the energy of the incoming chargedparticles. If Δβ is comparable to the intrinsic angle in the beam, itcontributes to the size of the spots, which are formed on an object,e.g. a wafer with resist, to be processed.

SUMMARY OF THE INVENTION

An objective of the current invention is to improve the apparatus forgenerating a plurality of beamlets for producing better-definedbeamlets.

A further objective of the present invention is to reduce blur ofbeamlets in an apparatus for generating a plurality of beamlets.

A further objective of the present invention is to reduce theaberrations induced by a collimator lens or lens assembly in anapparatus for generating a plurality of beamlets to a negligible value.

The invention therefore provides an apparatus for generating a pluralityof charged particle beamlets, comprising:

-   -   a charged particle source for generating a diverging charged        particle beam;    -   a converging means for refracting said diverging charged        particle beam;    -   a lens array comprising a plurality of lenses, wherein said lens        array is located between said charged particle source and said        converging means.

As a result of the position of the lens array, each beamlet is betterdefined. When the apparatus of the present invention is for instanceused in a multi-beam lithography system, the system can transfer apattern onto a substrate with higher beam current and resolution. Theapparatus will allow a microscopy system to produce an image of anobject with an enhanced resolution.

In an embodiment the lenses are electrostatic lenses, in fact lenslets,for converging a charged particle beamlet.

Each beamlet can result from a beam splitter array positioned before thearray of converging elements, which splits the beam of charged particlesfrom a source up in a plurality of beamlets. It can also be possible touse the array of converging elements to split the beam up into theplurality of beamlets.

In an embodiment of the apparatus of the present invention, theconverging means is adapted for refracting a diverging charged particlebeam into a substantially parallel charged particle beam for generatinga plurality of substantially parallel charged particle beamlets. Usingparallel beamlets makes the apparatus easier to control. Furthermore, inmany applications, especially lithography system, it is desirable tohave the beamlets impact the target substantially perpendicular to thetarget plane.

In an embodiment of the apparatus of the present invention, the chargedparticle source is arranged in a focal plane of the converging means. Inthis way, it is made sure that substantially parallel beamlets aregenerated.

In an embodiment of the apparatus described above, the lens array isarranged to project images of said source in the principal plane of theconverging means. This further reduces blur and makes it possible to usethe apparatus in high-resolution applications.

In most applications, the converging means will thus be a collimator,i.e. a lens or lens system or assembly, which refracts a diverging beaminto a substantially parallel, or even converging, beam. Usually, thismeans will comprise electrostatic lenses.

In an embodiment of the apparatus, it is furthermore provided withsplitting means for splitting said charged particle beam in a pluralityof charged particle beamlets. In this way, it was found possible toreduce the heat load of the lens array. Furthermore, better defined andseparated beamlets resulted. In an embodiment thereof, the splittingmeans comprises a spatial filter. In a specific embodiment, thesplitting means comprises an aperture array. This allows a simple androbust apparatus. In a further embodiment of the apparatus withsplitting means, the splitting means is located between said chargedparticle source and said lens array to split up said diverging chargedparticle beam into a plurality of charged particle beamlets. Thisprovides the possibility of obtaining well-defined beamlets and reducesthe heat-load of the lens array, thereby improving its performance.

In an embodiment of the apparatus with spatial filter, the spatialfilter is concave with respect to said source. In a preferred embodimentthereof, the curvature of said spatial filter has its focal pointsubstantially in the origin of the charged particle beam. In this way,it has proven possible to even further reduce blur and obtainbetter-defined beamlets.

In an embodiment, the lens array is concave with respect to said source.In a further embodiment thereof, the curvature of said lens array hasits focal point substantially in the origin of the charged particlebeam. This makes it possible to let all the beamlets pass through thecenter of a lens, thus reducing distortion of the beamlets even furtherby avoiding additional aberrations caused by said lens array.

In an embodiment of the apparatus with splitting means, the splittingmeans is aligned with said lens array for providing each lens with anindividual beamlet. In this way, each lens in the lens array receivesone beamlet created by the splitting means.

In an embodiment of the apparatus said lens array is an array ofelectrostatic lenses. In a further embodiment thereof, the chargedparticle beam is an electron beam. In another embodiment the chargedparticle beam is an ion beam.

In an embodiment of the apparatus, said lens array comprises an apertureplate, and means for defining a equipotential surface substantiallyparallel to the aperture plate at a distance from the aperture plate.Preferably at a different potential then the aperture plate itself.

In an embodiment of the apparatus, the aperture plate has a electricallyconducting surface and means for defining the electrostatic potential ofsaid surface. In this case, the resulting equipotential surfaces will beshaped as shown in the detail of FIG. 6. One embodiment is a (metal ormetal-covered) plate with through holes at the locations of beamlets.

In an embodiment of the apparatus, said means for defining anequipotential surface comprises a plate having a through hole at thelocation of the beam of beamlets, in particular a round hole with itscenter at the optical axis of the charged particle beam. A simpleembodiment is a (metal or other conducting metal or material coveredwith a conducting layer) plate with a round hole.

In an embodiment of the apparatus, said means for defining anequipotential surface substantially parallel to the aperture plate at adistance from the aperture plate are located between said source andsaid array of converging elements.

In an embodiment of the apparatus, said means for defining anequipotential surface substantially parallel to the aperture plate at adistance from the aperture plate are located between said array ofconverging elements and said converging means.

In an embodiment of the apparatus, said converging means comprises atleast one deflector array with deflectors aligned with the beamlets.Such a deflector can for instance be a plate with holes, provided with(two) opposite electrodes at the side of the holes or in the holes. Adetail is shown in FIG. 7.

In an embodiment of the apparatus, said converging means furthercomprises a controller for applying different voltages to the differentdeflectors of said deflector array.

In an embodiment of the apparatus, said controller is adapted forapplying voltages to each deflector of said deflector array fordeflecting a beamlet, with the controller adapted for setting thevoltages to have each deflector assert a deflecting effect proportionalto the distance of a deflector with respect the optical axis of thebeam. When the deflecting effect of each deflector is proportional toits distance from the optical axis of the beam, the net effect of allthe deflectors together can be deflecting the diverging beamlets in sucha way that their optical axes are almost parallel.

In an embodiment of the apparatus, said controller is adapted forapplying voltages to each deflector of said deflector array fordeflecting a beamlet, with the controller adapted for setting thevoltages to have each deflector assert a deflecting effect sufficientfor compensating aberrations of further converging devices of theconverging means. In this embodiment, the converging means comprise anelectrostatic lens acting as (main) collimator. These lenses usuallyhave spherical aberrations. When these aberrations are measured and thusexactly defined, the voltage of each deflector in the array can be setto compensate the local effect of the collimator.

In an embodiment of the apparatus, said converging means is anelectrostatic lens. In an embodiment of this apparatus, said lens arraycomprises an end plate providing a first electrode in said electrostaticlens. In an embodiment of this apparatus, it is further provided with asecond controller for applying a voltage to the electrodes of saidelectrostatic lens for operating said electrostatic lens substantiallyfree of spherical aberration.

In an embodiment of the apparatus, said array of converging elementscomprises means for defining a substantially planar equipotentialsurface at the location of said array. In an embodiment, one of thesurfaces of the array provided with conducting (metal) layer and avoltage is applied to it. There will then remain only the localdifferences in the equipotential surfaces depicted in FIG. 6, which haveonly a focussing effect on the beamlets.

In an embodiment of the apparatus, the apparatus further comprises afirst means for defining an equipotential surface between said array andsaid converging means.

In an embodiment of the apparatus, said first means comprises a plateprovided with a hole having a circumferential edge surrounding the beamor beamlets.

In an embodiment of the apparatus, said plate comprises a round holehaving its center aligned with the optical axis of the beam. In anembodiment, a metal plate with one through hole is provided, attached toa voltage source providing a (time) constant voltage.

In an embodiment of the apparatus, the apparatus is further providedwith means from applying an electrostatic potential to said plate.

In an embodiment of the apparatus, the apparatus further comprising asecond means for defining an equipotential surface after said convergingmeans and said.

In an embodiment of the apparatus, said second means comprises a plateprovided with a hole having a circumferential edge surrounding thebeamlets.

In an embodiment of the apparatus, said plate comprises a round holehaving its center aligned with the optical axis of the beam. Again, thiscan be a (metal) plate with a through hole.

In an embodiment of the apparatus, the apparatus further provided withmeans from applying an electrostatic potential to said plate.

The invention further pertains to a method of operating the variousvoltages described and in the attached drawings above in order to obtainthe effects disclosed.

The invention further pertains to a charged particle beam lithographysystem comprising the apparatus of the invention described above. Theapparatus is of the invention is capable of producing a very high numberof well-defined beamlets, making it possible to realize a lithographysystem having a high resolution, even smaller than 100 nm. A resolutionof below 20 nm seems possible. This high resolution can be combined witha high throughput (wafers/hour).

The invention furthermore relates to a substrate processed with thischarged particle beam lithography system.

The invention furthermore pertains to a charged particle beam microscopysystem comprising the apparatus of the present invention describedabove.

DRAWINGS

The invention will be further elucidated in the following embodiments ofa maskless lithography system according to the current invention, inwhich:

FIG. 1A shows an arbitrary source emitting a diverging beam,

FIG. 1B shows the source of FIG. 1B with a collimator positions in thebeam,

FIG. 2 shows the trajectory of a small portion of a charged particlebeam of FIG. 1B,

FIG. 3A shows the effect of the size of the source, resulting in anintrinsic opening angle α of the beam,

FIG. 3B shows the uncertainty Δβ resulting from charged particlespassing one collimation point,

FIG. 4 shows the positioning of the lens array according to the presentinvention,

FIG. 5 shows the apparatus of FIG. 4 with aperture array,

FIG. 6 shows the apparatus of FIG. 4, with electrostatic lenses effectof equipotential surfaces,

FIG. 7 uses a deflector array as collimator lens,

FIG. 8 uses equipotential plane for collimating,

FIG. 9 combines a deflector array with a collimator lens,

FIGS. 10A, 10B show the effect of a curved lens array.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A shows an arbitrary source 1 that emits a diverging chargedparticle beam 2. In a variety of systems not a diverging beam 2, but acollimated beam 3, i.e. a beam, which is substantially parallel, isdesired. Therefore a collimator lens 4 as shown (schematically) in FIG.1B or collimation assembly is positioned in the beam trajectory tocollimate the diverging charged particle beam 2. The collimation takesplace in the collimation plane or principal plane 5 of the collimator 4,denoted in FIG. 1B by the dotted line. After collimation the (almost)parallel charged particle beam can be split in a plurality of beamlets,which are subsequently focused on a target for pattern exposure,inspection, observation or other purposes.

In the collimation plane or principal plane 5, an incoming chargedparticle beam is refracted to create a collimated beam. FIG. 2 shows thetrajectory of a small portion of the diverging charged particle beam 2′which is refracted in the collimator plane at collimation point 6. Theangle between the initial direction and the final direction is denotedby β. In an ideal situation all emitted charged particles seem tooriginate from a singularity on the emission surface of the source 1.

However, in reality the charged particle path of a charged particleemitted from said source 1 is not as well defined as shown in FIG. 2. Infact the source size, seen from said collimation point 6 looking upwardto the source, is not a singularity. It has a finite size as shown inFIG. 3A. The finite size induces a finite angle α, called the intrinsicopening angle of the charged particle beam. Moreover, due to anintrinsic energy spread within the charged particle beam, thediffraction angle of each charged particle in the collimation plane 5 isnot as well-defined as shown in FIG. 2. The energy spread of the chargedparticles emitted by the source results in chromatic aberrations at thecollimation point, resulting in a dispersion Δβ in the deflection angleat the collimation point 6. This is shown in FIG. 3B, which shows adetail around point 6 of FIG. 3A.

The aberrations will become a problem when the beam is for instanceprojected on a target for exposure in lithography systems or forobservation in microscopy systems in which applications a very smallspot size is desired. Due to the aberrations the beamlet will getblurred and the spot size of the beamlet is no longer well-defined,which results in a blurry pattern or image. This is especially the casewhen angle Δβ comes in the order of the intrinsic opening angle α of thecharged particle beam. The source size or source image size scale with(α+Δβ)*l, wherein l is the distance between the collimation plane 5 andthe source 1. The influence of aberration Δβ on the focused spot size ofthe emitted charged particle beam scales with Δβ*l. Now if Δβ is of thesame order as or larger than the intrinsic opening angle α, asignificant reduction of resolution results.

The apparatus of the present invention provides a solution to overcomethe negative influences of the chromatic aberrations. The influence ofthese aberrations is avoided by positioning a lens array 7 having aplurality of lenses between said source 1 and said collimator lens 4 asis depicted in FIG. 4. In fact, said lens array 7 is positioned in sucha way that each lens of the lens array projects an image of said sourceon said collimation plane 5.

Consequently, the internal opening angle in the beam is determined bythe size of the beamlet in the lens array, d, and the distance betweensaid lens array and the collimation plane, f, as d/f. Thus, by choosingd and f, the intrinsic angle α can be made substantially larger than theaberration Δβ, and the blur in the system will therefore not increase.Thus the invention as described above provides the generation of aplurality of substantially parallel beamlets 9 by splitting andcollimating a diverging charged particle beam 2.

The invention can be further improved by adding additional splittingmeans into the system, preferably located between the source 1 and thelens array 7. The splitting means split the diverging charged particlebeam 2 in a plurality of diverging charged particle beamlets 11. In mostembodiments the splitting means comprise a spatial filter, preferably anaperture array 10 as is depicted in FIG. 5. Adding an aperture array 10at this position into the system provides a way to reduce the heat loadon the lens array 7. It furthermore enhances the performance of the lensarray 7 located behind (when following the optical pathway of thecharged particle beamlets) said splitting means.

The invention can be improved further by adding opening angle limitingmeans, preferably an aperture array, in the optical pathway of saidplurality of charged particle beamlets behind said collimator lens 4 orcollimator lens assembly. The opening angle limiting means are arrangedto limit the opening angle of the beamlets that have passed thecollimation plane and corrects for additional third order aberrationsinduced by said collimation lens 4.

For the lens array, any array of conventional charged particle lensescan be used. It is also possible, however, to use the embodiment shownin FIG. 6. In FIG. 6, three plates are installed, each at its ownpotential V1, V2 and V3. In fact, only the plane at V1 or at V3,together with the aperture plate 7 at V2, are actually needed in orderto have a lens effect. The plates at V1 and V3 have a hole large enoughto let the beam 2 pass without interfering with the beam 2.

Plate 7 is in fact the actual lens array. Plate 7 here is a plate withholes at the location where beamlets 8 should be created. In FIG. 6 anenlarged detail of one of the holes in plate 7 is shown. In thisenlarged view, the equipotential surfaces (indicated V′, V″, V′″) areadded, as well as the trajectory of two charged particles. Due to theshape of the equipotential surfaces, there will be a focussing effect onthe charged particle beamlets resulting from plate 7.

In another embodiment, which may even be combined with the above shownembodiments, shown in FIG. 7, instead of a conventional charged particlecollimating lens 4, a deflector array is used for collimating. In FIG.7, an enlarged detail of an embodiment of such a deflector array isshown. This deflector has holes at the location of the beamlets 8, andelectrodes at voltages V1-V6 (for the three deflectors shown). When thevoltages V1-V6 are carefully chosen, a deflection can be set in such away that the deflection is equivalent to the distance of the opticalaxis of charged particle beam 2. In such a way, it is possible to designan almost ideal collimator lens for this type of multi-beamlet systems.

It is even possible to design a multi-beamlet system which has virtuallyno spherical aberration. A layout for such a system is shown in FIG. 8.In this layout, lens array 7 is put on a Voltage V1. Macroscopically onthe scale shown in FIG. 8, the lens array 7 can be treated as anequipotential surface which is a plane. Furthermore, two plates areadded, one at a voltage V2 and the other at a voltage V3. These plateshave a (through) hole at the location of the beamlets, in order not tointerfere with the beamlets 8. When the dimensions of the plates and thevoltages V1, V2 and V3 are chosen in the right way, it is now possibleto create a collimator lens free of spherical aberration. It is known inthe field that such an aberration free lens can, according to Scherzerstheorem, only be created if one electrode of the lens is inside thebeam. In this particular apparatus, the inventors found out that is waspossible to used the aperture plate as an electrode which is inside thebeam. As a surprise, it seemed possible to use this type of layout forany multi beamlet system which uses charged particles. It can even beused for x-ray sources. Note that in FIG. 8, collimator lens 4 is onlyshown as a replacement schedule for the plates.

The embodiments shown in FIGS. 4 and 7 can be combined in a way shown inFIG. 9, in which way it leads to additional advantages. In this layout,the collimator lens 4 has a collimating effect with some sphericalaberration, depicted as an over collimation of the outside bealets inFIG. 9. Behind the collimator lens 4, a deflector array is now located.This deflector array has its deflectors aligned with the beamlets 9.When the voltages of the deflectors are properly set, is possible tocorrect the spherical aberration of the collimator lens 4 using onlyrelatively low voltages. In order to compensate for the sphericalaberration of the collimator lens 4, the voltage of each deflectorshould be set in such a way that its deflecting effect on the chargedparticle beamlet passing the deflector is proportional to the distanceof the deflector from the optical axis of the collimator lens 4 to thethird power.

The lens array 7 used in the present invention is preferably anelectrostatic lens array 12. FIG. 10A schematically shows an example ofsuch a lens array 12. In an embodiment, the lens array 12 comprises twoconducting plates with holes, positioned in close proximity to eachother wherein said holes in each plate are substantially aligned witheach other. An electric field is applied between the two plates byapplying a voltage difference V₁−V₂ between said plates as shown in FIG.10A.

Focusing a diverging charged particle beam may induce a further problem.Each beam passing through a planar electrostatic lens array 12 is notfocused correctly due to the fact that the incoming beamlets are notpassing the lenses perpendicular to the lens plane (i.e., parallel tothe lens axis). This complication affects the performance of the system,i.e. additional aberrations are introduced.

It was found that this complication could be avoided by applying aconvex lens array 13 with its inner surface facing the source asdepicted in FIG. 10B. When the convexity is well-matched with thedivergence of the beam, each beamlet passes the lens array 13 throughthe holes substantially perpendicular to the surface of both plates.

The charged particle beam that is used can be any charged particle beamknown in the art, but preferably an electron beam or ion beam. Theinvention can be used in a lithography system or microscopy system. In alithography system the invention provides a way to pattern a substrateto be patterned with enhanced resolution, since the spot size of thecharged particle beam is kept small. Furthermore in microscopy systemsobjects can be imaged with higher resolution.

It is to be understood that the above description is included toillustrate the operation of the preferred embodiments and is not meantto limit the scope of the invention. From the above discussion, manyvariations will be apparent to one skilled in the art that would yet beencompassed by the spit and scope of the present invention.

1. An apparatus for generating a plurality of charged particle beamlets,comprising: a charged particle source, for a diverging charged particlebeam generated by said source; a lens array comprising a plurality oflenses; splitting means comprising a spatial filter comprising anaperture array, for splitting said charged particle beam in a pluralityof charged particle beamlets; said splitting means being aligned withsaid lens array for providing each lens with an individual beamlet. 2.The apparatus of claim 1, wherein the system comprises a convergingmeans for refracting said diverging charged particle beam.
 3. Theapparatus of claim 2, wherein said lens array is located between saidcharged particle source and said converging means.
 4. The apparatus ofclaim 1, wherein said spatial filter is located between said chargedparticle source and said lens array to split up said diverging chargedparticle beam into a plurality of charged particle beamlets.
 5. Theapparatus of claim 1, wherein the charged particle beam is an electronbeam and said lens array is an array of electrostatic lenses.
 6. Theapparatus according to claim 1, wherein said lens array comprises anaperture plate.
 7. The apparatus of claim 6, further comprising meansfor defining an equipotential surface at a distance from said apertureplate at a different potential than said aperture plate itself.
 8. Theapparatus of claim 7, wherein said equipotential lines are definedsubstantially parallel to said aperture plate.
 9. The apparatus of claim7, wherein the aperture plate has a electrically conducting surface andmeans for defining the electrostatic potential of said surface.
 10. Theapparatus of claim 7, wherein said means for defining an equipotentialsurface comprises a plate having a through hole at the location of thebeam of beamlets, in particular a circular hole having its center at theoptical axis of the charged particle beam.
 11. The apparatus of claim10, wherein said means for defining an equipotential surface are locatedbetween said source and said lens array.
 12. The apparatus of claim 10,wherein said means for defining an equipotential surface are locatedbetween said array of converging elements and said converging means. 13.The apparatus of claim 2, wherein said converging means comprises atleast one deflector array with deflectors aligned with the beamlets. 14.The apparatus of claim 2, wherein said converging means is anelectrostatic lens.
 15. The apparatus of claim 1, wherein said chargedparticle beam is an electron beam.
 16. The apparatus of claim 1, whereinsaid charged particle beam is an ion beam.
 17. A charged particle beamlithography system comprising the apparatus of claim
 1. 18. A substrateprocessed with the charged particle beam system of claim
 1. 19. Acharged particle beam microscopy system comprising the apparatus ofclaim
 1. 20. The apparatus according to claim 1, wherein the convergingmeans is adapted for refracting a diverging charged particle beam into asubstantially parallel charged particle beam for generating a pluralityof substantially parallel charged particle beamlets.
 21. The apparatusaccording to claim 1, wherein said charged particle source is arrangedin a focal plane of said converging means.
 22. The apparatus accordingto claim 1, wherein the lens array is arranged to project images of saidsource in the principal plane of said converging means.